The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for diffusion-weighted MRI near metallic implants or devices.
Diffusion-weighted imaging (“DWI”) is an MRI method for imaging certain brain and spinal cord pathologies, such as infarction (e.g., brain cell death) caused by ischemic stroke; nerve demyelination; and cytotoxic edema. Currently, DWI MRI relies on echo planar imaging (“EPI”) pulse sequences to induce proton magnetization in tissue water molecules as part of the imaging process. However, imaging with EPI pulse sequence is highly susceptible to the effects of metallic implants or devices in the subject. As a result, the acquired images are often unrecognizable due to image distortions caused by metallic object-induced magnetic field perturbations.
To overcome the problem of the distortions caused by metallic implants or devices, multi-spectral imaging (“MSI”) was developed. In general, MSI combines multiple MRI magnetic source measurements into a single set that is used to construct an undistorted image. MSI techniques have been successful in improving imaging of tissues near metal implants; however, these techniques are based on fast spin echo (“FSE”) imaging and are therefore not inherently supportive of diffusion-weighting. In particular, the addition of diffusion lobes to an FSE sequence violates the CPMG condition, resulting in a rapid decay of the amplitude of the echo train. Additionally, FSE sequences do not solve the problem of obtaining high quality DWI images of brain, spinal cord, and other tissues if metal implants are present.
There remains a need then for the ability to perform diffusion-weighted MRI in regions near metallic objects, such as metallic implants or other devices.